Hybrid circuits are customarily used in various places as a signal interface between a bidirectional two-wire or transmission path and the individual unidirectional sections or unidirectional transmission paths of a four-wire line. In telephone carrier systems, for example, a hybrid circuit is used as a junction between a two-wire drop or subscriber loop and a four-wire telephone carrier channel. Hybrid circuits are also used in the telephone set itself to couple the telephone's receiver and transmitter to a two-wire line.
The most commonly used hybrid circuit design is a hybrid transformer known as a hybrid coil. This type of hybrid circuit typically has a four winding transformer to provide a two-wire port, separate transmit and receive ports, and a balance network. The transformer windings are arranged in such a manner that signals imposed on the two-wire port are coupled to the transmit port and signals imposed on the receive port are coupled to the two-wire port, but not to the transmit port.
Hybrid transformers of the type described above have been in use for many years. They have, however, a number of drawbacks including relatively large size and high cost. To avoid these problems, various forms of transformerless or electronic hybrids have been proposed in the past such as the ones described in U.S. Pat. No. 4,004,109 which issued to F. S. Boxall on Jan. 18, 1977 and U.S. Pat. No. 4,064,377 which issued to J. F. Regan on Dec. 20, 1977.
In the Boxall patent signals are coupled between the two and four-wire lines by an arrangement of current mirrors. Such current mirrors, however, tend to be relatively expensive and are not readily available, low cost, off-the-shelf items.
The Regan patent avoids the use of current mirrors and hence the relatively high expense associated with such componentry. Regan's electronic hybrid circuit, however, is particularly designed for relatively high voltage supplies such as -48 VDC and is not suitable for low voltage supplies (e.g., -16 VDC) because of the voltage drops that are created by connecting terminating resistances in series with the conductors of the two-wire line. Additionally, the Regan hybrid circuit requires a carefully designed differential amplifier circuit to achieve the balance necessary for adequate common mode rejection.
The present invention has none of the foregoing drawbacks and instead is designed to be particularly suitable for use with low voltage supplies as well as affording the advantage of being manufacturable from inexpensive, off-the-shelf components. Additionally, the present invention affords several other advantages as will become apparent from the following summary and description of the invention.